Pluto was excluded from the list of planets in the Solar System because it no longer meets the criteria for being a planet under the official definition adopted by the International Astronomical Union (IAU) in 2006.
📜 What happened?
In 2006, the IAU introduced a formal definition of what constitutes a “planet.” According to that definition, a celestial body must meet three criteria to be considered a planet:
- It must orbit the Sun.
✅ Pluto does this. - It must be spherical (have enough gravity to pull itself into a round shape).
✅ Pluto does this too. - It must have “cleared the neighborhood” around its orbit.
❌ Pluto does not do this.
🚫 The key issue: “Clearing its neighborhood”
This third criterion means that the planet must be gravitationally dominant—it must have either consumed or flung away other objects in its orbital zone.
- Pluto shares its orbital space with many other objects in the Kuiper Belt—a region full of icy bodies beyond Neptune.
- Because it hasn’t cleared this region, Pluto was reclassified as a “dwarf planet.”
💡 Why the controversy?
- Many scientists and the public felt an emotional attachment to Pluto as the 9th planet.
- Some argue the IAU’s definition is too narrow or even arbitrary.
- Others believe the dynamical context (“clearing the orbit”) is essential for distinguishing a true planet.
🪐 So what is Pluto now?
Pluto is officially a “dwarf planet,” along with others like:
- Eris
- Haumea
- Makemake
- Ceres (in the asteroid belt)
If you’re interested, I can also show how Pluto’s orbit compares with other planets or Kuiper Belt Objects in a diagram.
